Cooling cover for annealing furnaces



Dec. 25, 1956 G. J. CAMPBELL ET AL 2,775,441

' COOLING COVER FOR ANNEALING FURNACES 5 Sheets-Sheet 1 Filed Feb. 2, 1.955

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ATTOh E\ Gena/y (Hal Lewis G- 006 Dec. 25, 1956 a. J. CAMPBELL ET AL 2,77

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COOLING COVER FOR ANNEALING FURNACES 5 Shee ts-Sheet 3 Filed Feb. 2, 1953 VI," flllld rIIIII/IlIl/IlfliI/llIl/ V INVENTORS Gaorye J 0 bell and Lewis .6. 00 6am.

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ATTO EY United States Patent COOLING COVER FOR ANNEALING FURNACES George J. Campbell, Sparrows Point, and Lewis G. Coscia, Lochearn, Md., assignors to Bethlehem Steel'Company, a corporation of Pennsylvania Application February 2, 1953, Serial No. 334,656

1 Claim. (Cl. 263-50) This invention relates in general to a method and apparatus for heat-treating metals, and more particularly to a method and apparatus for accelerating the rate of cooling or coiled steel strips or the like during the process of annealing.

In the usual method of annealing coiled steel strip, a base of refractory lined structural steel is equipped with one or more refractory pedestals which support the charged coils under gas tight inner covers which have the primary purpose of preventing oxidation. A bell type furnace consisting of a refractory lined steel shell provided with inner heating elements is then placed on the base by means of an overhead crane, and is removed when the heating and soaking cycle has been completed. The furnace is then placed over another base containing a cold charge of steel and the heating and soaking cycle is'again started. The first hot charge of steel, from which the furnace was removed, but with the inner cover .still in place, is allowed to cool until a predetermined temperature is reached. A prepared atmosphere is constantly circulated inside the inner cover during the heating, soaking and cooling cycles to prevent oxidation of the charge at elevated temperatures and for better heat transfer between the charge and the cover. The heat is thus transmitted through the inner cover during the cooling cycle and dissipated into the surrounding atmosphere.

The time and temperature of the heat treating cycle will vary according to the'end use of the steel and the size and gauge of the charge. For example, an average charge of commercial grade sheet coils, representing a total weight of about 525 tons on an eight pedestal base, may be raised to 1300 F. in 36hours, held at that temperature for 12 hours, and then allowed to cool to 250 F. before the inner covers are removed. In general, the length of the conventional cooling cycle is approximately twice the length of the heating and soaking cycle, so that full utilizationof-each-furnace will require at least three bases, even in open buildings with ample access to outside air for natural draft circulation and cooling. Where the buildings lack an adequate supply of outside air the ratio of bases to furnaces may have to be raised to four to one, or even higher, depending upon the analysis and end use of the steel. The result is that furnaces are sometimes made idle by lack of available bases, and any increase in annealing capacity by installing additional furnaces involves a large capital outlay for additional bases and the necessary building area.

Furthermore, workmen loading or unloading bases or performing other duties adjacent to masses of steel of such size at temperatures up to 1300 F. required protection from the excessive radiated heat. Adjacent furnaces must also be protected from premature ignition of pre-mixed gas due to high ambient temperatures developed in pipe lines, fire checks, etc., from the heat radiation.

It is therefore desirable to limit the cooling cycle to the shortest possible time in order to reduce the ratio 2,775,441 Patented Dec. 25, 1956 of bases to furnaces to a practical minimum, and also to surround the charges with effective heat shields. In some instances, water has been sprayed on the inner covers to help dissipate the heat. The disadvantages of this method are that water cannot be used until the temperature of the charge has been reduced to approximately 700 F. to 900 F. and even then part of the water is converted into vapor by the heat, creating very unpleasant working conditions, and the remaining water must be collected and drained away which is not practicable with sand sealed inner covers.

One object of this invention, therefore, is to provide means to reduce the cooling cycle to the shortest possible time by forcing air against the outside of the inner covers, thereby absorbing the heat transmitted from the coils at a more rapid rate than is possible with natural draft alone.

7 Another object of the invention is to provide a heat shield which will protect workmen and furnace equipment from the excessive heat radiation in the vicinity of cooling charges of steel.

Other objects, purposes and advantages of the inven- Fig. 5 is a vertical detail sect1on taken on the line 55 of Fig. 4;

Fig. 6 is a vertical detail section through the double wall of the cover on the line 6-6 of Fig. 1;

Fig. 7 is a horizontal detail section of a corner of the cover, taken on the line 77 of Fig. 2; s

Fig. 8 is a horizontal detail section of a buckstay, on the line 8-8 of Fig. 3; and

Figs. 9 and 10 are curves illustrating'the slow rate of cooling coiled sheets in still air in the conventional manner (without cooling cover), as compared with the use of the cooling cover for all (Fig. 9) .or even only the last part (Fig. 10) of the cooling cycle.

Referring to the drawings in detail, the furnace base 1 is conventionallyprovidcd with a foundation of structural members 2 supporting a metal floor plate 3, upon which in turn are supported refractory blocks 4 bearing a plurality of pedestals (not shown). On each pedestal rests a charge of coiled strip (not shown) under a cylindrical inner cover 5. The base 1 illustrated in Fig. l is provided with a total of eight inner covers 5, arranged in parallel rows of four inner covers each, although other arrangements involving either more or fewer inner covers are obviously possible according to convenience. Beneath a central orifice (not shown) in each pedestal is a vertically disposed radial blower (not shown) in a housing 6, provided with an inlet 7 for admitting a specially prepared non-oxidizing atmosphere. Upright channels 8 and bulb angles 9 welded around the outer edges of the floor plate 3 hold sand or other suitable powdered refractory 10 in which a depending sealing strip (not shown) on the annealing furnace is embedded during the heating and soaking cycle to prevent infiltration of air. At diagonally opposite corners of the base are set upright guide posts 11 of extra-heavy metal pipe.

The cooling cover 12 comprises a rectangular frame top and bottom edges. Welded securely to the top edge I-beams 14 are cross-beams 16 to which is attached a very strong and rigid lifting bail 17 to allow safe movement by an overhead crane. The bottom edge I-beams are provided with inner bearing angles 18 and web plates 19 to permit the cover to rest on the mating channels 8 (Fig. 6), and with outer corner brackets 20 whereon are bolted funnel-shaped sockets 21 (Figs. 4-5) to receive the guide posts 11 for insuring correct positioning of .the cover on the base.

Referring to Figs. 7 and 8, enclosing double side and end walls with an intervening air space are provided by inner sheets 22 and outer sheets 23 of suitable thickness and with their side edges welded to the vertical .buckstays 13 and inner and outer corner angles 24 and 25.

The top of the cooling cover 12 is leftopen 'to the air. Around the lower edges of the cover there is a circumferential row of open louvres 26 formed by a plurality of upwardly tilted vanes 27 welded to parallelogramshaped side plates 28 secured between adjacent pairs of buckstays 13. A plurality of electric motors 29 driving large axial flow fans 30 (ten being shown), each directed upward about 10 from horizontal and located .to force a very strong stream of cooling air between and around the inner covers 5, are mounted in suitably spaced openings in the side and end walls of the cooling cover. Conventional flexible cords and plugs (not shown) are provided to connect the fan motors 29 to electric power supply lines, which may also include ordinary variable control means for regulating motor speeds if desired.

In operation, after the end of the heating and soaking period, the annealing furnace is removed and the cooling cover is placed around the inner covers on the base, the electrical connections are plugged in and the fans are started.

After the charge has been cooled to proper temperature (generally about 250 F.), the cooling cover is removed and placed in operation over a hot charge on another base, while on the base first used the inner covers are lifted off and the fully annealed charge is unloaded.

The objective of the air streams generated by the fan plus the air drawn in through the louvres is to absorb and dissipate the heat transmitted through the inner'covers in the shortest possible time consistent with the required metallurgical properties of the charge.

Accordingly, the motors 29 and fan blades 30 should be of adequate size, speed and design to deliver the necessary volume and velocity of cooling air with minimum power consumption. For example, with an 8-pedestal installation of the type shown in the drawings, we have found in practice that ten 6-bladed fans of 24-inch diameter, each driven at 1725 R. P. M. by a 1 /2 horsepower motor and delivering 8800 cubic feet of air per minute, will cool an average charge of about 525 tons of covered coiled steel strip from 1300 F. to 250 F. at approximately the rate shown in Figs. 9 and 10.

Referring further to Figs. 9 and 10, and taking initial 48 hour periods of combined heating and soaking time as constant in each case, it will be noted that a typical cooling time of 96 hours per base under ordinary conditions has been reduced to 62 hours with full time use of the cooling cover, and to 66 hours in part time use including an initial 12 hour period of slow heat loss without the cooling cover, thus showing a reduction in cooling time of 31.5% to 35.5%. The resultant savings of 30 to 34 hours are equivalent to reductions from the former total base time of 144 hours of approximately 20% to 24%, thus permitting proportional increases in annealing capacity to be achieved through the use of more furnaces without additional bases or floor space, as only 2.17 bases instead of 3 bases will be required for each furnace.

The air space enclosed by the double walls has the advantages of reducing the normal heat transfer between the inner and outer wall sheets 22 and 23, and also, in the event of power failure, of permitting sufiicient natural flow of air therethrough to keep said sheets and the supporting frame members from being unduly overheated.

The cooling covers are thus able to function also as very effective heat shields protecting adjacent personnel, furnaces and plant installations from excessive radiated heat.

Although we have described our invention as applied to cooling coils of steel strip under inner covers, the same principle may be applied to cooling coils of tinplate and black plate, wire and/ or flat sheets, either of ferrous or non-ferrous metals, when annealed under covers filled with a prepared atmosphere in a bell type furnace.

Therefore, we do not wish to be limited narrowly to the exact and specific details disclosed above, but may also use such substitutions, modifications or equivalents thereof as are included within the scope and spirit of the invention or pointed out in the appended claim.

We claim:

An .outer cover for accelerating the cooling of hot steel strip' or the like sealed under inner covers on an annealing furnace base, comprising double side and end walls forming a chamber open at top and resting on the basesurrounding the inner covers, vertical buckstays disposed at regular intervals within said side and end walls and defining air circulating passages therebetween, rigid circumferential top and bottom edge members attached to said buckstays, a plurality of open louvres having upwardly inclined vanes positioned near the bottom edges between adjacent buckstays, upwardly directed fans on motors mounted on the side and end walls above the louvres for forcing strong air streams inwardly between and around the inner covers to withdraw heat therefrom, and means for controlling the motor and fan speeds to regulate the rate of heat withdrawal in accordance with a predetermined time period.

References Cited in the file of this patent UNITED STATES PATENTS 1,752,857 Seeds Apr. 1, 1930 2,401,918 Elder et a1. June 11, 1946 2,503,764 Ott Apr. 11, 1950 2,657,036 Nichols Oct. 27, 1953 FOREIGN PATENTS 252,405 Great Britain Dec. 9, 1926 

